1. Field of the Invention
This patent specification relates borehole and surface seismic technology. More particularly, this patent specification relates to systems and methods for making seismic measurements from borehole and surface deployed seismic sources and receivers.
2. Background of the Invention
The majority of oil exploration drilling decisions are based upon the seismic survey map. However, in surface seismic surveys the velocity information along the depth are derived from the surface receiver array. That information is less and less accurate when the depth of interest increases. Due to the above reason the depth as well as the shape of the oil reservoir is often distorted and inaccurately mapped as the depth of the reservoir increases. Therefore, oil companies risk landing their wells at the wrong places and waste significant amount of capital investment. There are increasing needs for refining the seismic map while drilling. Because of the drilling situation, one can measure the velocity directly from the bit to the surface by using a pair of seismic source and receiver. However, the traditional wireline-based borehole seismic measurements that provide such a measurement are not transparent to the drilling process and often require a significant amount of rig time. Therefore, it is challenging to perform such measurements inexpensively and easily while drilling. Thus there is a need to provide the industry with a new way of measuring seismic velocity accurately while drilling with a minimum interference to the current drilling practices. With accurate velocities at depth geophysicists and geologists in the oil companies can update their seismic map to make accurate decisions in developing their reserves. Such new technology could provide a better map and make their jobs easier and more successful.
Tools such as Schlumberger's Seismic VISION tool are designed to solve the logistical problem of wireline borehole seismic intervention and provide accurate velocity measurement while drilling. Due to the lack of cable connection in the drilling process, the firing of a surface seismic source with down hole receivers is carefully synchronized. A significant challenge in such technologies is synchronizing the clocks between the firing of a surface seismic source and recording of down hole receivers. A pair of highly accurate clocks are used, one up hole and one down hole, and schedule the surface firing at a preprogrammed schedule and start the downhole receiver measurements according to the same schedule. For example, see U.S. Pat. No. 6,606,009 which discloses a dual mode oscillator crystal with a first output having a frequency related to temperature and a second output having a frequency substantially stable with respect to temperature; and U.S. Pat. No. 6,912,465 which discloses methods for determining the drift of a clock adapted for subsurface disposal.
Another drawback of this type of method is that one loses the flexibility of the data acquisitions and the measurement itself creates unnecessary operational difficulties both on the service and drilling crews. In addition, the highly accurate clocks are generally expensive to build and maintain. Furthermore, due to the limited bandwidth of communication from the borehole tool to the surface, it can be challenging to transmit the downhole-recorded seismic data to the surface.